Petroselinic acid (18:1.sup..omega.12) is an unusual fatty acid that is found primarily in seeds of Umbelliferae (Apiaceae), Araliaceae, and Garryaceae species (Kleiman, R. & Spencer, G. F. (1982) J. Am. Oil Chem. Soc. 59, 29-38). This fatty acid is also found in Nigella sativan, a member of the family Ranunculaceae and Aucuba japonica, a member of the Cornaceae family. This fatty acid may comprise more than 80% of the total fatty acid of seeds but is virtually absent in leaves and other tissues of these plants (Ellenbracht, F., Barz, W. & Mangold, H. K. (1980) Planta 150, 114-119; Dutta, P. C. & Appelqvist, L. A. (1991) Plant Sci. 75, 177-183).
The structure of petroselinic acid differs from that of oleic acid (18:1.sup..omega.9cis), a common plant fatty acid, by the position of its double bond. Because of the presence of unsaturation at the .sup..omega.12 carbon atom, petroselinic acid is of potential industrial significance. Through chemical cleavage at its double bond, petroselinic acid can be used as a precursor of lauric acid (12:0), which is a component of detergents and surfactants, and adipic acid (6:0 dicarboxylic), which is the monomeric precursor of nylon 6,6. Petroselenic acid when esterified to triacylglycerols is also resistant to hydrolysis by pancreatic lipase. Therefore, such oils may have value as low calorie fats. Because this fatty acid has not previously been available in bulk quantities, not all of the potential industrial or food uses are known at present. Therefore, it may be advantageous to develop, by genetic engineering, lines of higher plants which accumulate petroselinic acid.
The pathway for petroselinic acid biosynthesis has not been previously determined. Preliminary results from a variety of [.sup.14 C]-labelling studies suggest that this fatty acid is the product of an acyl-acyl carrier protein (acyl-ACP) desaturase (FIG. 1; Cahoon, E. B. & Ohlrogge, J. B. (1991) INFORM 2, 342; unpublished data). The only acyl-ACP desaturase to have been previously characterized in plants is the .omega.9 stearoyl-ACP (18:0-ACP) desaturase (EC 1.14.99.6) which catalyzes the conversion of 18:0-ACP to 18:1.sup..omega.9 -ACP (Nagai, J. & Bloch, K. (1968) J. Biol. Chem. 243, 4626-4633; Jaworski, J. G. & Stumpf, P. K. (1974) Arch. Biochem. Biophys. 162, 158-165; McKeon, T. A. & Stumpf, P. K. (1982) J. Biol. Chem. 257:12141-12147). This reaction is readily assayable in tissue extracts of most plants using [.sup.14 C]18:0-ACP and cofactors including ferredoxin, NADPH, and ferredoxin-NADPH reductase (Jaworski, J. G. & Stumpf, P. K. (1974) Arch. Biochem. Biophys. 162, 158-165; McKeon, T. A. & Stumpf, P. K. (1981) Methods Enzymol. 71, 275-281). By analogy, it is thought that petroselinic acid is synthesized by .omega..sup.12 desaturation of stearoyl-ACP or by .omega..sup.12 desaturation of palmitoyl-ACP with subsequent two carbon atom elongation (FIG. 1). However, the in vitro synthesis of petroselinic acid from [.sup.14 C]acyl-ACPs, including [.sup.14 C]18:0-ACP, has not been detected using seed extracts of the Umbelliferae species coriander and carrot (unpublished data). Lack of such an assay complicates any attempt to characterize the biosynthetic pathway or to purify the acyl-ACP desaturase postulated to be involved in petroselinic acid synthesis.